Here are some thoughts, not complete answers though...
Q1 - As Chris mentioned, if you get the remote set-up correct, you can certainly have 9.1.3 (or 8.2) on one logical server to talk to EG 4.1 and 9.2 on a different logical server to talk to EG 4.2. I think the biggest problem you may discover with this approach is that EG 4.2 projects CANNOT be opened in EG 4.1. And once you open an older project into EG 4.2 and save it in the new version, you can't ever go back to using it in EG 4.1. Of course, you could do a "save as" rather than a save to avoid this problem.
Q2 - EG repository support was discontinued, and my understanding is there's really no "easy" button to turn an EG repository into a SAS 9.2 metadata repository. I believe you have to manually create a SAS 9.2 metadata repository to resemble your old EG repository. If your 4.1 projects were talking to an EG repository, you'll have to alter things like the library assignments before they'll correctly point to a 9.2 metadata repository. The two primary ways to do make these changes are by using the Migration Wizard or using the Project Maintenance window (found under the Tools menu in EG 4.2).
Q3 - If you haven't used EG 4.2 before, there were a fair number of changes to the GUI. I do a lot of work in EG 4.2 from the submenus, which are completely new. You can use most of your old EG 4.1 tricks too, but you'll notice differences in the interface. One of the big ones to get used to is if you double-click on a task icon, it opens up the results rather than the task dialog. The task order didn't change a lot, but you'll discover all the tasks are now under a Task menu, rather than separate menus. For an experienced EG 4.1 user, I'd plan a "breaking in" period to get those users used to 4.2. You can probably show them the basic changes in less than an hour, but getting comfortable will take a bit more time.
Q4 - Offhand, I'd say your biggest time commitments will be getting your 9.2 metadata repository set up and your EG projects migrated. I will let others speak to this a bit more since I haven't gone through a serious migration personally.
In case you're wondering, getting 9.2 and 4.2 eventually running in your environment is going to be worth it. There are some really nice features in EG 4.2 and having that 9.2 metadata repository is so helpful for sharing and securing your information.
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